Non-invasive tracking of bones for surgery

ABSTRACT

There is described a device for use with a position sensing system for tracking a bone in a reference coordinate system, the device comprising a fabric removably and non-invasively attachable to a bone and having a plurality of reference markers distributed thereon, the reference markers being one of passive and active devices recognized by the position sensing system and positioned and oriented in the reference coordinate system with respect to a fixed reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional PatentApplication No. 60/832,151 filed on Jul. 21, 2006, the contents of whichare hereby incorporated by reference, and is related to U.S. patentapplication of the same title filed concurrently herewith.

TECHNICAL FIELD

The present invention relates to the field of computer-assisted medicalprocedures, and more specifically, to bone tracking and positioning incomputer-assisted surgery (CAS) systems.

BACKGROUND

Computer-assisted surgery (CAS) makes use of references fixed to thepatient using pins inserted into the bones of the limbs or the pelvis.These pins, inserted into the bones before or during the surgery, are ofdifferent diameter sizes and can cause pain after the surgery. They arean extra step to the surgery, exclusively because of the navigationsystem. Also, the insertions of the pins into the bone may causeweaknesses of the bone that can then more easily be fractured.Infections may also occur as for any entry point at surgery.

Furthermore, the length of the pins is sometimes obtrusive to thesurgeon who may cut them to a length better adapted to his movementduring the surgery. The cut is also perceived as an extra annoying step;its end may be sharp and hazardous to the personnel working around thesurgery table.

The pins are time-consuming and invasive. Therefore, there is a need foran improvement in this area.

SUMMARY

The systems and methods described herein reduce the invasiveness and thetime required when using pins in order to reference the bones of apatient during surgery.

In accordance with a first broad aspect of the present invention, thereis provided a method for determining a position and orientation of abone in space, the method comprising: removably attaching in anon-invasive manner at least three base units to a skin surface coveringthe bone, each of the base units having a reference marker attachedthereto; measuring a distance between each of the base units and thebone; registering position and orientation readings of the referencemarkers with respect to a fixed reference in the reference coordinatesystem; and determining the position and orientation of the bone usingthe position and orientation of the reference markers and the distancebetween the base units and the bone.

The base units can be attached using a non-toxic adhesive, or providedon a fabric, such as a sock, belt, underwear, shorts, or any other typeof apparel that can be worn on the body at the appropriate location. Theultrasound can be integrated inside the base units, or an external probecan be applied to each base unit to determine the distance from the skinsurface to the bone.

In accordance with a second broad aspect of the present invention, thereis provided a method for tracking a bone in a reference coordinatesystem, the method comprising: removably attaching in a non-invasivemanner a piece of fabric around a bone, the fabric having a plurality ofreference markers distributed thereon; registering position andorientation readings of the reference markers with respect to a fixedreference in the reference coordinate system; and determining a positionand orientation of the bone using the readings.

The piece of fabric can be anything that can be attached to the body,such as a sock or belt. The reference markers can be optical, RadioFrequency (RF), (electro)magnetic, ultrasound, or any other known typeof passive or active reference markers used in computer assistedsurgeries.

In accordance with a third broad aspect of the present invention, thereis provided a system for determining a position and orientation of abone in space, the system comprising: a set of at least three base unitsremovably and non-invasively attachable to a skin surface covering abone, each of the base units having a reference marker attached thereto;at least one ultrasound device adapted to emit an ultrasound wave,receive an echo of the ultrasound wave off of a surface, and record atime measurement for the echo; a fixed reference positioned in thereference coordinate system and used to identify a position of thereference markers in the reference coordinate system; a position sensingdevice adapted to register position and orientation readings of thereference markers in a reference-coordinate system; and a processingunit receiving the position and orientation readings and the timemeasurement, translating the time measurement into a distancemeasurement, and determining the position and orientation of the bone.

In accordance with a fourth broad aspect of the present invention, thereis provided a system for tracking a bone in a reference coordinatesystem, the system comprising: a fabric removably and non-invasivelyattachable to a bone and having a plurality of reference markersdistributed thereon; a fixed reference positioned in the referencecoordinate system and used to identify a position of the referencemarkers in the reference coordinate system; a position sensing deviceadapted to register position and orientation readings of the referencemarkers; and a processing unit receiving the position and orientationreadings and determining a position and orientation of the bone.

In accordance with a fifth broad aspect of the present invention, thereis provided a device for use with a position sensing system to registerposition and orientation in a reference-coordinate system, the devicecomprising a set of at least three base units removably andnon-invasively attachable to a skin surface covering a bone, each of thebase units having a reference marker attached thereto, the referencemarkers being one of passive and active devices recognized by theposition sensing system and positioned and oriented in the referencecoordinate system with respect to a fixed reference, the base unitsadapted to measure a distance between the skin surface and the bone inconjunction with an ultrasound component.

In accordance with a sixth broad aspect of the present invention, thereis provided a device for use with a position sensing system for trackinga bone in a reference coordinate system, the device comprising a fabricremovably and non-invasively attachable to a bone and having a pluralityof reference markers distributed thereon, the reference markers beingone of passive and active devices recognized by the position sensingsystem and positioned and oriented in the reference coordinate systemwith respect to a fixed reference.

In this specification, the term “reference marker” is intended to meanan active or passive marker, such as an emitter or a reflector. The term“fixed reference” may also refer to any active or passive device, with aknown position in the reference coordinate system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a front view of a pelvic bone with three base units attachedon the skin surface;

FIG. 2 is a cross-sectional view of an elongated bone, such as a femur,with three base units attached on the skin surface;

FIG. 3 is a front view of an elongated bone, such as a femur, withmultiple pairs of base units provided thereon;

FIG. 4 is a cross-sectional view of an elongated bone, such as a femur,with a plurality of reference markers around the bone;

FIG. 5 is a cross-sectional view of an elongated bone, such as a femur,with a single pair of reference markers around the bone;

FIG. 6 is an illustration showing a piece of fabric with referencemarkers thereon attached to a leg;

FIG. 7 is a block diagram of an embodiment of a system of the presentinvention used with a set of base units; and

FIG. 8 is a block diagram of an embodiment of a system of the presentinvention used with a fabric having reference markers attached thereto.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION

FIG. 1 illustrates a pelvic bone 10 covered by the skin 12. Three baseunits 14 are attached to the skin 12. The base units 14 may be attachedusing a medical adhesive, i.e. non-toxic for the patient, or it can beprovided on a piece of fabric that is worn by the patient during thesurgery. For example, a pair of snug-fitting shorts or underwear(undergarment) having the base units 14 attached thereto are worn by thepatient.

One base unit 14 is attached to each crest of the hip (anterior superioriliac spines), and a third base unit 14 is attached to the pubis area(pubic symphysis) of the pelvis. These locations on the pelvis arechosen for having minimum distance between the out-skin 12 and the bone10. The base units 14 may be positioned at other locations on the pelvicbone, without being restricted to these locations in particular.

Reference markers 16 are present in/on each base unit. These referencemarkers 16 may be active or passive, optical, RF, (electro-)magnetic, orother. In FIG. 1, optical reflective reference markers are illustrated.These three points define the pelvic coordinate system. The positionsensing system used with the CAS system will register the position andorientation in space of the pelvic bone with respect to eitherpre-operative images of the patient, such as CT-scans, fluoroscopy,x-rays, etc, or with respect to any type of intra-operativereconstruction of the bones illustrated on an interactive displaydevice. A fixed reference present in the coordinate system and having aknown position is used to position and orient the pelvic bone in space.

In one embodiment of the present invention, the distance of each baseunit 14 on the out-skin 12 to the bone 10 is measured using anultrasound probe that is applied to each base unit 14. The ultrasound,which is a transducer, emits an ultrasound wave and measures the time ittakes for the wave to echo off of a hard surface (such as bone) andreturn to the transducer face. Using the known speed of the ultrasoundwave, the time measurement is translated into a distance measurementbetween the base unit and the bone located below the surface of theskin. In another embodiment, an ultrasound device is integrated intoeach base unit 14. The measurement is done by either triggering itmanually, or automatically. In one embodiment, the measurement isrepeated at regular intervals. The measurements are constantly beingtransferred to the CAS and the position and orientation of the bone 10in space is updated. The measurement of the distance from the base unitto the bone may also be done using alternative imaging means, such asfluoroscopy. A metal reference is positioned on the skin surface andused with a fluoroscopy system to identify the bone surface. It ispossible to make the measurement from the metal reference to the bonesurface on the fluoroscopic image

Once the distance between the base unit 14 and the bone 10 is known, anestimation of the possible displacement of the base unit 14 on theout-skin relative to the bone 10 can be done and the bone can then beregistered to the reference system. In the case of the integratedultrasound device within each base unit 14, it becomes possible tomeasure in real-time variations in distance between the base units 14and the bone 10 during the surgery.

FIG. 2 is a cross-sectional view of a bone 18, such as a femur, a tibia,or any other bone having an elongated form. The base units 14 arepositioned around the bone 18 on the out-skin 12. Similarly to the caseof the pelvis 10, the base units 14 may be glued onto the body 18, or apiece of fabric having the base units 14 attached thereto is placed onthe body in a snug-fitting manner. For a limb, a sleeve, sock, or beltmay be used. The base units 14, each having reference markers 16 aretherefore distributed around the bone. If the reference markers 16 areof the optical type, than only the reference markers 16 in theline-of-sight of the position sensing system will be registered. In thecase of a limb, this may mean that only half of the reference markersare visible. The other half can be extrapolated using the readingsobtained from the visible markers. For other types of reference markers,such as RF emitters, all reference markers on the bone will generate areading in order to register the position and orientation of the bone inspace.

When tracking the orientation and position of an elongated bone, theproblems encountered due to cutaneous movement can be resolved byplacing a single reference marker directly on the bone at the beginningof the surgery. If that reference marker is an ultrasound, the soundemitted by the reference marker can be captured by the base units on theskin. Therefore, the invasive pins are replaced by an ultrasound link,and the only invasive part is the incision that would have been made inany case.

FIG. 3 is a front view of the bone shown in cross-section in FIG. 2. Thebase units 14, including the reference markers (not shown), are placedon the out-skin 12 of the bone 18. The distance between the bone 18 andthe out-skin 12 is measured using ultrasound. The possible variation ofdistance between the base and the bone during the surgery can bemeasured, either once at the beginning or in real-time during thesurgery.

The anatomical axis 20 of the bone 18 can also be determined. Theultrasound probe is not necessary to determine the anatomical axis 20.Pairs of reference markers 16 on the out-skin 12 are positioned suchthat they are substantially facing each other, as illustrated in FIG. 4.The mid-point between the measured position of each marker in a pairwill constitute a point on the anatomical axis 20.

A minimum of one pair, i.e. two reference markers 16 face-to-face, isneeded to determine the anatomical axis, as illustrated in FIG. 5, ifused in combination with a single reference marker placed at a distalend of the bone. Alternatively, two pairs of reference markerspositioned face-to-face could also be used to determine the anatomicalaxis 20 by providing at least two points on the axis 20.

In one embodiment of the present invention, a belt 20 having ultrasoundemitters thereon is attached around a leg of a patient, as illustratedin FIG. 6. A fixed reference is provided in the reference system, forexample on a cutting block positioned on the knee, or attached to thepelvis bone (not shown). The reference markers on the belt 20 arereferenced to the fixed reference, for example by using a pointer andapplying it to the reference markers on the belt 20. This can also bedone automatically without a pointer. The ultrasound emitters measurethe distance between the skin and the bone, and the position andorientation of the bone in space is determined.

In another embodiment, a belt having reference markers (not necessarilyultrasound emitters) is attached around a leg of the patient. Thereference markers are positioned such that there are pairs of markerssubstantially facing each other. The anatomical axis of the bone isdetermined by locating the middle point between a pair of markers andforming a line from these points along the bone. The position of thebelt in space is determined using the reference markers on the belt. Atleast one pair of reference markers are needed to determine theanatomical axis, if the reading from a single reference marker providesa position from which more than one point on a line can be determined.

FIG. 7 illustrates a system in accordance with one embodiment of thepresent invention. A processing unit 30 and a position sensing system 32are coupled with a set of at least three base units 14, each base unit14 having a reference marker 16 thereon. The reference markers 16 on thebase units 14 are used to position the bone within a referencecoordinate system. The position sensing system, as is known in the art,will use either active or passive devices as markers. The orientationand position of the bone in space can be determined using theinformation obtained from the reference markers 16 and using the knownposition of the fixed reference 34 in the coordinate system. Inaddition, an ultrasound device 36 is used to measure the distancebetween the surface of the skin and the bone underneath the surface. Byupdating this measurement, a more precise positioning of the bone isobtained.

The base units may be provided with a non-toxic adhesive on a surfaceand stuck directly onto the skin. Alternatively, a fabric mounted withthe base units is attached to the skin, as is described above. Theultrasound device is either integrated into each base unit, or usedexternally to the base units by applying it manually to each base unit,in the form of a probe, for example.

An alternative embodiment of the system is illustrated in FIG. 8. Aprocessor 40 and a position sensing system 42 are coupled to a fabric 20removably and non-invasively attachable to a bone and having a pluralityof reference markers 16 distributed thereon, and a fixed reference 44positioned in the reference coordinate system and used to identify aposition and orientation of the reference markers in the referencecoordinate system.

The reference markers 16 on the fabric 20 may be distributed on thefabric in a variety of ways. One such way is to have at least tworeference markers substantially facing each other when the fabric iswrapped around a bone. One or more rows of pairs of markers can beprovided on the fabric, as is illustrated in FIG. 5.

The processor is preferably a general purpose computer equipped withsoftware that allows it to compute the location of a bone surface fromthe information obtained by the reference markers and ultrasound device.The results may be displayed on a screen or monitor to be visualized bya user or operator. The information may be used in conjunction withother known registration techniques to assist in pre-operative orintra-operative procedures. The components of the system may need to becalibrated using standard procedures known to the person skilled in theart.

While illustrated in the block diagrams as groups of discrete componentscommunicating with each other via distinct data signal connections, itwill be understood by those skilled in the art that the preferredembodiments are provided by a combination of hardware and softwarecomponents, with some components being implemented by a given functionor operation of a hardware or software system, and many of the datapaths illustrated being implemented by data communication within acomputer application or operating system. The structure illustrated isthus provided for efficiency of teaching the present preferredembodiment. The embodiments of the invention described above areintended to be exemplary only. The scope of the invention is thereforeintended to be limited solely by the scope of the appended claims.

1. A method for tracking a bone in a reference coordinate system, themethod comprising: removably attaching in a non-invasive manner a beltaround a bone, said belt having a plurality of reference markersdistributed thereon, and positioning at least two of said referencemarkers such that they are substantially facing each other when saidbelt is around said bone; registering without imagery position andorientation readings of said reference markers with respect to a fixedreference in said reference coordinate system; determining withoutimagery a position and orientation of said bone using said readings,including calculating an anatomical axis from readings of said referencemarkers on each side of said bone by determining a mid-pointtherebetween; and repeating said registering and said determining toupdate said position and orientation readings while tracking said bone.2. A method as claimed in claim 1, wherein said removably attachingcomprises positioning a plurality of pairs of facing reference markersalong said anatomical bone to increase a precision reading of saidanatomical axis.
 3. A method as claimed in claim 2, wherein saidreference markers are ultrasound emitters used to measure a distancebetween said belt and said bone.
 4. A method as claimed in claim 1,wherein said registering position and orientation readings of saidreference markers with respect to a fixed reference comprisesreferencing to a fixed reference attached to a separate bone.
 5. Asystem for tracking a bone in a reference coordinate system, the systemcomprising: a fabric removably and non-invasively attachable around abone and having a plurality of reference markers distributed thereon,said reference markers being distributed on said fabric such that atleast two of said reference markers are substantially facing each otherwhen said fabric is wrapped around said bone; a fixed referencepositioned in said reference coordinate system and used to identify aposition of said reference markers in said reference coordinate system;a position sensing device adapted to register position and orientationreadings of said reference markers; and a processing unit receiving saidposition and orientation readings as continuously updated anddetermining a position and orientation of said bone without imageryusing said readings, and wherein said processing unit is adapted tocalculate an anatomical axis from said readings of said referencemarkers on each side of said bone by determining a mid-pointtherebetween.
 6. A system as claimed in claim 5, wherein said referencemarkers are distributed to form at least two rows of pairs of facingreference markers.
 7. A system as claimed in claim 5, wherein at leasttwo of said plurality of reference markers are ultrasound devicesadapted to emit an ultrasound wave and record an echo of said ultrasoundwave off of a surface.
 8. A system as claimed in claim 5, wherein saidfixed reference is on a cutting block in said reference coordinatesystem.
 9. A system as claimed in claim 5, wherein said fixed referenceis on a pelvis bone in said reference coordinate system.